As the second week of his testimony, Kaing Guek Eav alias Duch testified that the Communist Party of Kampuchea killed their own blood—killing their own cadres and soldiers. For the rest of the day, Duch testified about the operation at S-21 when he was a chief during Democratic Kampuchea. When asked about photography at this security office, Duch said that all, except special prisoners, were photographed. "Special prisoners were not photographed because the Party intended to keep secret;" said Duch in the courtroom when asked by International Deputy Co-Prosecutor William Smith, "Son Sen was the one who decided which prisoners were considered special who were high-ranking members of the Party." Duch added that prisoners were photographed for record and that in case the prisoners managed to escape, those photographs could be used to track them down.

When asked about the fate of prisoners after being admitted to S-21, Duch said that all prisoners were tortured and that the minister of commerce Koy Thuon was the only one who was not tortured because Duch's supervisor instructed him not to do so. Duch admitted that he interrogated Koy Thuon four times and his confession was hundreds of pages in length. The confession was sent to higher echelon for review. Later, Koy Thuon was executed on the order of Son Sen who was then the minister of national defense and who directly supervised Duch.

According to Duch, after August 17, 1977, Nuon Chea took over and directly controlled S-21 after Son Sen went to the battle field as tension with Vietnam intensified at the Eastern border.

During the conflict between the two nations, after Vietnamese soldiers were captured by the government of Democratic Kampuchea, according to Duch, S-21 deputy chief Mam Nay who could speak fluent Vietnamese, was responsible for interrogating those captives and those voices were recorded, which later was broadcasted on national radio of Democratic Kampuchea.